1. Field of the Invention
This invention relates to an improvement in a dual mode drum brake device which functions as a leading-trailing (LT) type brake device during the service brake operation and functions as a duo servo (DS) type brake device during the parking brake operation.
2. Prior Art
A drum brake as described in the Japanese Provisional Patent Publication No. 10-110758 is known to be an example of a dual mode drum brake device, the summarized operation of which is shown in FIGS. 13-16. Summary of the drum brake device in its inactivated state is explained with reference to FIG. 13. A pair of brake shoes 111, 112 are positioned to face each other, wherein the fixed cylinder device 113 is positioned between adjacent ends of the brake shoes 111, 112 at one end while the fixed anchor 114 is positioned between adjacent ends of the brake shoes 111, 112 at the other end.
A pivot lever 116 is pivotally supported on the brake shoe 111 by a pin 115 between both ends of the brake shoe 111 while a parking brake lever 118 is pivotally supported on the brake shoe 112 by a pin 117 at the end portion of the brake shoe 112 adjacent to the anchor 114.
A first strut 119 is extended between a pivot section of the brake lever 118 and one free end of the pivot lever 116 adjacent to the anchor 114 while a second strut 120 is extended between the other free end of the pivot lever 116 as well as an upper end of the brake shoe 111 adjacent to the cylinder device 113 and the upper side of the brake shoe 112 corresponding to the free end of the pivot lever 116 and the brake shoe 111.
When a service brake operates via a foot brake pedal, the cylinder device 113 is pressurized to advance two pistons housed therein moving both brake shoes 111, 112 outward from positions shown in FIG. 13 to rotate and separate the same brake shoes apart, pivoting each at an abutment point with the anchor 114. Accordingly, the brake shoes 111, 112 frictionally engage with the inner circumferential surface of the brake drum, not shown in the figures, to generate the braking force.
The drum brake device operates as a leading-trailing type drum brake where one of the brake shoes 111, 112 becomes a leading shoe in relation to the rotational direction of the brake drum, thereby generating a self-servo property, and the remaining brake shoe becomes a trailing shoe in relation to the rotational direction of the brake drum without a self-servo property.
The parking brake operation is described while referring to the accompanying FIGS. 14-16. If a hand lever, not shown in the figures, is operated to activate the parking brake, the brake lever 118 is pulled in the direction of arrow W in FIG. 14 via members such as a parking brake cable to rotate accordingly pivoting around a pin 117. As such, the brake lever 118 pushes the strut 119 to the right in FIG. 14 so as to rotate the pivot lever 116 in a counterclockwise direction pivoting around the pin 115.
Such rotation of the pivot lever 116 pushes the strut 120 to the left in FIG. 14 so as to move the brake shoe 112 outwardly pivoting around the abutment point with the anchor 114 as shown in the same figure.
A reaction force generated during the above-operation acts as xcex1 to the right in FIG. 14 via the pin 115, which pushes the brake shoe 111 in the same direction. An upper shoe return spring, not shown in the figures, is stretched between the brake shoes 111, 112 adjacent to the cylinder device 113 while a lower shoe return spring, not shown, is stretched between the brake shoes 111, 112 adjacent to the anchor 114, wherein a moment around the pin 115 when combining the effects of a spring force of the lower shoe return spring and a distance from the pin 115 to the lower shoe return spring is larger than that of the upper shoe return spring and a distance from the pin 115 to the upper shoe return spring, thereby the above reaction force a acting on the brake shoe 111 via the pin 115 urging to move the brake shoe 111 outward, pivoting at the abutment point with the anchor 114 as shown in FIG. 14. A clearance xcex2 is created in an engagement section between the second strut 120 and the brake shoe 111. Both pistons housed in the cylinder device 113 separate apart to follow the brake shoe 111, 112 respectively by a coil spring compressed therebetween.
Due to the rotational movement shown in FIG. 14, the brake shoes 111, 112 frictionally engage with the inner circumferential surface of the brake drum.
Regarding an explanation as to the braking operation when a counterclockwise rotational force xcex3 as shown in FIG. 15 is acted on the brake drum, the brake shoe 111 is trailed by the rotating brake drum from the position in FIG. 14 in order to fill up the clearance xcex2, also as shown in FIG. 14, to collide and contact with the second strut 120. The rotational force of the rotating brake drum is received by the anchor 114 via the strut 120 and the brake shoe 112. When a clockwise rotational force xcex4 as shown in FIG. 16 is acted on the brake drum, the brake shoe 112 is trailed by the rotating brake drum and the second strut 120 shifts from the position in FIG. 14 to the right in order to fill up the clearance xcex2 so as to collide and contact with the brake shoe 111. The rotational force of the brake drum is transmitted through the strut 114 and the brake shoe 111 and is received by the anchor 114.
As is evident from the above-description, when either the clockwise or counterclockwise rotational force of the brake drum is received by the anchor 120, the rotational force is transmitted from one of the brake shoes 111, 112 to the anchor 114 via the second strut 120 and the remaining brake shoe 111 or 112, thereby effecting the parking brake application. Therefore, both brake shoes 111, 112 function as leading shoes having self-servo property regardless of the rotational directions of the brake drum, the drum brake device functions as a duo servo type drum brake.
A conventional dual mode drum brake device uses the strut 120 as a member to transfer the rotational force of the brake drum between both brake shoes 111, 112 during the parking brake operation. The strut 120 needs to be extended between both brake shoes 111, 112. Because of the restriction in its layout, the strut 120 is required to be positioned inwardly relative to the cylinder device 113, and a distance between a drum center, indicated as an X in FIG. 13, and the strut 120 becomes shorter, which tends to increase the self-servo property of the brake shoe 111 or 112 at the primary side during the brake drum rotation.
Here, a relational expression between a brake factor BF representing an effectiveness of the parking brake and braking torque may be: (braking torque)=(brake shoe input)xc3x97(effective radius of drum)xc3x97(BF). For the above-described conventional dual mode drum brake device, as a coefficient of friction xcexc of a brake lining becomes larger, the rate of increase in the brake shoe factor of the primary brake shoe becomes larger, and therefore, input from the primary brake shoe to the secondary brake shoe quickly increases; the total brake factor BF by adding the brake shoe factor of the secondary brake shoe onto that of the primary brake shoe quickly increases as shown in FIG. 12; and the brake factor BF over-increases within the practical range of the coefficient of friction xcexc of the lining. Accordingly, when considering a stronger brake effectiveness during the parking brake operation, the dual mode drum brake device should be designed stronger than the brake device bearing the strength during the service brake operation and a secure mounting strength of the device, thereby causing drawbacks of increasing the weight and cost of the device.
Further, for the conventional dual mode drum brake device, the primary brake shoe in relation to the rotational direction of the brake drum rotates so that the end of the primary brake shoe adjacent to the cylinder device 113, as shown in FIGS. 15 and 16, proceeds into the cylinder device 113 to fill up the clearance xcexc as appeared in FIG. 14 during the parking brake operation, and the piston corresponding to or facing that brake shoe end is greatly pushed relative to the amount of the brake shoe movement approaching the other piston. Under this condition, if the parking brake is released while applying the foot brake pedal, a pedal stroke becomes larger for the approached distance of both pistons, thereby causing a problem of disconcerting the driver""s feeling for the pedal.
Further, in order to avoid supporting the braking torque of the primary brake shoe during the parking brake operation by a brake cable via the brake lever 118, the brake lever 118 may not be configured to directly push the strut 120, and the parking brake lever 118 needs to be pivoted at the end portion of the brake shoe 112 adjacent to the anchor 114. Therefore, the conventional device may not use the typical structure for a leading-trailing type drum brake device in which the parking brake lever 118 is pivoted at the end portion of the brake shoe 112 adjacent to the cylinder device 113, which reduces the flexibility in its application and causes difficulty in routing and arranging the brake cable because of existing obstacles such as a suspension device.
It is an object of this invention to provide a dual mode drum brake device to resolve all of the above problems.
Further, it is an object of this invention to substantiate a dual mode drum brake device with a forward-pull type parking brake lever and a cross-pull type parking brake lever in the same arrangement as a publicly known leading trailing type drum brake, which facilitates the substitutional use of a leading trailing type drum brake. Moreover, it is an object of this invention to substantiate a dual mode drum brake device in which a forward-pull type parking brake lever and a cross-pull type parking brake lever are utilized just like a conventional dual mode drum brake device, which increases flexibility of brake cable arrangement and routing.
It is another object of this invention to provide a dual mode drum brake device with an added value, wherein the joint member coupling both pistons within the cylinder device may have a shoe clearance adjustment function, or the shoe clearance adjustment function may be conducted automatically, and/or the strut may have a parking brake stroke adjustment function, or the parking brake stroke adjustment function may be conducted automatically.
To that end, a dual mode drum brake device of this invention comprises: a pair of brake shoes positioned to face each other and to be frictionally engaged with an inner circumferential surface of a brake drum, each brake shoe has a first brake shoe end and a second brake shoe end respectively, a cylinder device positioned between a pair of the first adjacent brake shoe ends while a fixed anchor positioned between a pair of the second adjacent brake shoe ends; the cylinder device acts during a service brake operation to advance pistons at a pair of cylinder ends so that the pistons spread the brake shoes apart about abutment points between the pair of the second adjacent brake shoe ends and the anchor to frictionally engage with the inner circumferential surface of the brake drum, a pivot lever is pivotally positioned between ends of one brake shoe; the pivot lever moves along a plane surface which makes a right angle to a brake drum rotational axis, an anchor side strut is extended between a first free end of the pivot lever and the second brake shoe end of the other brake shoe while a cylinder side strut is extended between a second free end of the pivot lever and the first brake shoe end of the other brake shoe and positioned inwardly relative to the cylinder device so that an operating force of a parking brake lever is transmitted to the one brake shoe and the other brake shoe both as an outward force in a radial direction of the brake drum, thereby enabling the operating force of the parking brake lever to separate the brake shoes apart, wherein a force transmitting member is extended between the pair of the first adjacent brake shoe ends and positioned outwardly relative to the cylinder side strut; the force transmitting member transmits a rotational force of the brake drum while a parking brake action is engaged from either brake shoe positioned at a primary side relative to the brake drum rotational direction to a remaining brake shoe positioned at a secondary side relative to the brake drum rotational direction, and the force transmitting member is composed of the two pistons and a joint member extended between the pistons of the cylinder device so as to enable the pistons and the joint member to move integrally while the parking brake action is engaged.
Further, the parking brake lever may engage with the other brake shoe at the first brake shoe end, and the parking brake lever engages with an end of the cylinder side strut at the other brake shoe side.
Still further, the parking brake lever may engage with the other brake shoe at the second brake shoe end, and the parking brake lever engages with an end of the anchor side strut at the other brake shoe side.
Still further, the parking brake lever may engage with the first free end of the pivot lever, and the parking brake lever engages with an end of the anchor side strut at one brake shoe side.
Still further, the joint member may be a shoe clearance adjustment device adjusting clearances between the brake shoes and the inner circumferential surface of the brake drum.
Still further, the shoe clearance adjustment device may be an automatic shoe clearance adjustment device responding to an amount of outward movement of the brake shoes and automatically adjusting the clearances.
Still further, at least one of the cylinder side strut and anchor side strut may include a stroke adjustment device adjusting an amount of the stroke of the parking brake lever.
Still further, the stroke adjustment device may be an automatic stroke adjustment device responding to an amount of outward movement of two brake shoes and automatically adjusting the stroke.
Yet, further, the parking brake lever engages with the second free end of the pivot lever, and the parking brake lever engages with an end of the cylinder side strut.
In this invention, a member transmitting the rotational force of the brake drum between both brake shoes during the parking brake operation is positioned at the outermost side of the drum brake device. Therefore, the distance from the member transmitting the rotational force to the drum center becomes longer, and the self-servo property of both brake shoes during the brake drum rotation is set lower than the conventional device, thereby maintaining the lowest minimum required.
Accordingly, unlike the conventional dual mode drum brake device designed to bear the strong parking brake and requiring to secure a mounting strength of the device itself, the parking brake effectiveness as described herein only needs to clear the minimum requirement, thereby realizing a lighter dual mode drum brake device as well as reducing the cost of the device. During the parking brake operation, if one piston is pushed, the other piston follows to keep a constant clearance between the two pistons. Therefore, if the parking brake is released while the foot brake pedal is applied, the pedal stroke does not become larger, which does not give any disconcerting pedal feeling to the driver.
Further, the brake lever may be pivotally supported on either end of the brake shoe, which does not set any restriction in the brake lever arrangement, and a typical structure for a leading trailing type drum brake device may be utilized, thereby increasing an applicability of the device and not requiring any change in the brake cable arrangement as in a conventional drum brake device.
Furthermore, the above coupling member may be a shoe clearance adjustment device adjusting the shoe clearance between both brake shoes and the inner circumferential surface of the brake drum or the shoe clearance adjustment device may be an automatic shoe clearance adjustment device automatically adjusting the shoe clearance in response to the amount of outward movement of the brake shoe, thereby creating an additional value.
Furthermore, a stroke adjustment device adjusting the stroke amount of the brake lever may be employed in one of the first strut or the second strut or the stroke adjustment device may be an automatic stroke adjustment device automatically adjusting the stroke amount of the brake lever in response to the outward movement of the brake shoe, thereby creating an additional value.